The present invention relates to a semiconductor memory device and, more particularly, to technology which is especially effective when used in a multi-port memory contained in a frame member of a digital processor system as a personal computer or work station.
Multi-port memories are known which include random access ports and serial access ports. Such memories can have a raster computing function and a flashing light function or the like, and are effective as a frame memory. Digital processor systems such as a personal computer or work station are also known, which include a display device using a CRT (i.e., cathode ray tube) or the like, and a frame memory having a serial access memory such as the aforementioned multi-port memory for transmitting a series of image data to the display device.
On the other hand, it is becoming the current practice that personal computers or work stations are given the so-called "resume function", in which the displayed content of a display device is held in the frame memory even after interruption of the power supply so that the displayed content before the interruption may be instantly resumed. Digital processor systems having such a resume function are required to enable at least the serial access memory constituting the frame memory to be backed up by a battery.
An example of a multi-port memory having random access ports and serial access ports is disclosed in Japanese Patent Laid-Open No. 293791/1988. On the other hand, a digital processor system having the resume function is disclosed, for example, on pages 18 and 19 of "DynaBook J-310OSS DynaBook Guide" issued by Toshiba Corp., on Jul. 6, 1989.
The multi-port memory of the prior art described above is not provided with the so-called "backup model" so that it cannot be backed up by a battery. As a result, the existing digital processor systems having the resume function adopt a method of using an ordinary static RAM (i.e., random access memory) having the battery backup mode. However, the ordinary static RAM is not equipped with the serial access ports nor is it given special functions such as a raster computing function or a flashing light function for matching the frame memory effectively. This raises a problem that the bit rate of the image data is limited to restrict the function as a frame memory, although the resume function can be achieved. Incidentally, an example of a static RAM having the battery backup mode is disclosed on pages 250 to 259 of "Hitachi IC Memory Data", issued in September, 1989 by Hitachi, Ltd.